B-C-N compounds

A surprising variety of calcium compounds with combinations of mixed B, C or N anions is known, with some of them listed in Table 8.1. Their structures may contain anionic networks, or very often, triatomic anions with 16 valence electrons, and other anionic species. Some examples of Ca-(B,C,N) compounds are displayed in Fig. 8.1, and will be briefly discussed here. 

Fig. 8.1 Crystal structures of some calcium-(B,C,N) compounds (a) CaBe (LaBg type) (b) CaB4 xCx (LaB4 type) (c) CaB2C2 (corresponding with LaB2C2) (d) Can(CN2)2N6 (e) Ca3(BN2)N (f) Ca3Cl2C3 (g) Ca3Cl2(CBN).

S.2 Problems and Pilfalls of some Calcium Compounds with (mixed) B,C,N Anions 123 Table 8.1 Some Ca-(B,C,N) and Ca-CI-(B,C,N) compounds. 

I 8 Current State on (B,C,N) Compounds of Calcium and Lanthanum Energy... 

Mixed B-C-N compounds of lanthanum maybe subdivided into La-(BNx), La-(BCx), and La-(CNx) compounds (Fig. 8.15). The chemistry of lanthanide nitridoborates has been developed in some detail and some properties were studied. But still more work is necessary, especially in the field of quaternary Ln-metal-(BNx) compounds. 

As a defined ternary B-C-N compound is not known, other starting materials are to be considered in order to synthesize one of the, yet unknown, La-B-C-N compounds. Solution chemistry routes were successfully performed with tetra-cyanoborates to synthesize the compounds A[B(CN)4] with monovalent A = Li, Ag, Cu cations, and M[B(CN)4]2 with divalent M = Hg, Cu, Zn, Mn cations [47]. 

Cubic BC2N. Hetero-diamond B C—N compounds have recently received a great interest because of their possible applications as mechanical and optical devices. The similar properties and structures of carbon and boron nitrides (graphite and hexagonal BN, diamond, and cubic BN) suggested the possible synthesis of dense compounds with all the three elements. Such new materials are expected to combine the best properties of diamond (hardness) and of c-BN (thermal stability and chemical inertness). Several low-density hexagonal phases of B,C, and N have been synthesized [534] while with respect to the high-density phases, different authors report contradictory data [535-538], but the final products are probably solid mixtures of c-BN and dispersed diamonds [539]. 

TABLE 9 Magnetic properties of the homologous R-B-C(N) compounds RB15 5CN, RB22C2N, and RB28.5C4 (R = Er, Ho). Tf is defined as the peak temperature of the zero field cooled (ZFC) susceptibility... 

As noted in Section 9, the structures of the R-B-C(N) compounds (Figure 21) are homologous to that of boron carbide which exhibits typical p-type characteristics. Boron carbide is the limit where the number of boron icosahedra and C-B-C chain layers separating the metal layers reaches infinity (i.e. no rare earth layers). It has been speculated that the 2 dimensional metal layers of these rare earth R-B-C(N) compounds are playing a role for the unusual n-type behavior, but the mechanism is not yet clear. 

Recent investigations have revealed that the intrinsic behavior of RB28.5C4 is also n-type (Mori et al., 2008a). Very small inclusions of boron carbon "B4C" can cause the p-type behavior previously observed in some samples (Mori and Nishimura, 2006). The origin of the striking n-type behavior observed in the homologous R-B-C(N) compounds is not completely resolved yet but indicated to pertain to the two-dimensional rare earth layers (Mori et al., 2008a). 

Although the presently obtained absolute values of power factors still need to be improved, these compounds may offer a n-type boride counterpart to the well known p-type boron carbide which is one of the few thermoelectric material viable for extremely high temperature use (1500 K), and further research on the R-B-C(N) compounds is merited. 

The B-C-N compounds are attractive materials due to their unique properties associated with their layered, graphite-like structures [246, 247]. Such compositions can be deposited by the CVD of a mixture of BCI3, ammonia and a hydrocarbon (usually acetylene) at temperatures between 700-1700 °C [247-249]. A compound with the composition BC2N has been obtained by the CVD of a mixture of BCI3 and acetonitrile... 

The most striking discovery is the series of rare earth borocarbonitrides (or R-B-C(N)) compounds which are found to be the long awaited n-type counterpart to p-type boron carbide as high-temperature TE materials. ... 

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